1. Field of the Invention
The present invention relates to push-on switches mounted on operating parts of various electronic devices.
2. Background Art
As electronic devices have been smaller, lighter, thinner, and more functional in recent years, it has been strongly desired to reduce the size and thickness of push-on switches mounted on their operating parts.
A conventional push-on switch will be described as follows with reference to FIGS. 14 to 16. FIGS. 14 and 15 are a sectional view and an exploded perspective view, respectively, of the switch. FIG. 16 is a sectional view showing an operating condition of the switch. As shown in FIGS. 14 to 16, the push-on switch includes case 1, which is made of synthetic resin and has an open-top recess. The recess has an inner bottom surface in which central fixed contact 2 and two outer fixed contacts 3 symmetric with respect to central fixed contact 2 are exposed. Case 1 includes terminals 2A and 3A, which are connected to central and outer fixed contacts 2 and 3, respectively, and led out from case 1.
The push-on switch further includes movable contact 4, which is made of elastic sheet metal and is surface-treated to have high conductivity on its bottom surface. Movable contact 4 has an upwardly convex dome shape with an open bottom, and is housed in the recess of case 1 as follows. The bottom of the outer periphery of movable contact 4 is mounted on outer fixed contacts 3, and the bottom surface of the top of the dome thereof faces the top surface of central fixed contact 2 with a space therebetween.
The push-on switch further includes protective sheet 5, which is made of an insulating film and has adhesive 6 on its bottom surface. Protective sheet 5 covers the recess of case 1 and is adhesively fixed to case 1 via adhesive 6.
The conventional push-on switch thus structured operates as follows.
The user applies a compressive force to the top of the dome of movable contact 4 from above protective sheet 5. When the compressive force exceeds a predetermined force, the center of the dome of movable contact 4 is elastically inverted to a downwardly convex shape as shown in FIG. 16 with a click feel. As a result, the bottom surface of the center of movable contact 4 comes into contact with central fixed contact 2 located beneath it. This provides electrical continuity between central and outer fixed contacts 2 and 3 via movable contact 4, thereby turning on the switch between terminals 2A and corresponding terminals 3A.
When the user releases the compressive force, the center of the dome of movable contact 4 elastically returns to the upwardly convex dome shape shown in FIG. 14 with a click feel, so as to move away from central fixed contact 2. As a result, the switch between terminals 2A and corresponding terminals 3A is turned off.
Examples of a conventional technique related to the present invention are shown in Japanese Patent Unexamined Publications Nos. 2003-297175 and 2002-63823.
In the above-described conventional push-on switch, fixed contacts 2, 3 and terminals 2A, 3A are insert-molded to case 1. Therefore, when case 1 has a small thickness, its thin portion is likely to be insufficiently filled with synthetic resin during insert molding, thereby making it difficult to make the push-on switch thin and compact. Moreover, the insert-molded members are heat-shrunk, causing a small gap in the contact area between the insert-molded members and the synthetic resin. As a result, it is difficult for case 1 to have high waterproofness.